If the time that ignition current flows through the ignition coil is determined solely by the pulse width of the above-mentioned pulses, the time that current flows prior to ignition may be too short to allow it to reach the minimum value required for ignition during increasing engine speed. It has therefore been proposed (for example in DE-OS No. 1,539,178 to which U.S. Pat. No. 3,482,560 corresponds) that, under conditions of increasing engine speed, the time that the series switch is in the conductive state between successive ignition signals is increased, while the time it is in the non-conductive state is decreased. In this known system, the control is derived from an a-c generator furnishing a signal having a particular wave shape. However, the geometric shape of the rotor of this generator must be empirically derived and such a generator is, in general, difficult to manufacture. Further, as mentioned above, an a-c generator is required.